1. Field of the Invention
This invention relates to electronic circuitry, and more particularly to circuitry for protecting bipolar-complementary metal oxide semiconductor (BiCMOS) circuits during power transitions.
2. Description of the Prior Art
Many circuits require protection during power transitions, that is, during the short time interval while power is being turned on or off. Circuits having both bipolar and MOS devices particularly require protection because the bipolar devices and the MOS devices have different characteristics during power transitions. For example, data in a BiCMOS static RAM is stored in a plurality of flip-flops which are individually selected for reading or writing information. In such circuits, the voltage levels present during a power transition may be sufficient to turn-on the CMOS transistors, but leave the bipolar transistors in the circuit essentially off. The output levels of the substantially off bipolar transistors appear to the remainder of the circuit as legitimately "high". The unintended high outputs may cause the CMOS devices in the circuit to enter undefined states. In a BiCMOS memory device, these undefined states can cause data loss or multiple selects with resultant high current and potential damage to the BiCMOS chip.